Blood pressure estimation apparatus and blood pressure estimation method

Abstract

To provide a non-invasive blood pressure estimation apparatus, which can accurately estimate systolic blood pressure from blood flow sound of a dialysis patient and continuously estimate systolic blood pressure by continuously picking up the blood flow sound. A blood pressure estimation apparatus creates a standard pulse curve by relating a start point and end point of a rising phase of a pulse wave to diastolic blood pressure and systolic blood pressure, respectively, creates a correspondence curve between blood flow sound power and estimated blood pressure by contrasting the standard pulse curve with the blood flow sound power curve obtained from blood flow sound at a shut site, with the two curves plotted on the same time axis, derives a systolic blood pressure estimation linear function from the correspondence curve, inputs a measured maximum value of the blood flow sound power into the linear function, and thereby estimates systolic blood pressure.

The invention claimed is: 
     
       1. A blood pressure estimation apparatus which estimates blood pressure of a subject using a standardization mode and an estimation mode, comprising:
 blood flow sound power waveform calculation means for determining a blood flow sound power waveform from blood flow sound of the subject in the standardization mode; 
 blood pressure measuring means for measuring reference systolic blood pressure and reference diastolic blood pressure of the subject in the standardization mode; 
 standard pulse curve storing means for storing a standard pulse curve whose time axis and pulse pressure axis have been standardized such that times and pulse pressures at a start point and an end point of a rising phase of a pulse wave measured in advance will be constant, in the standardization mode; 
 first computing means for determining a correspondence curve which represents a relationship between blood flow sound power and estimated blood pressure from relationships among the blood flow sound power waveform, the reference systolic blood pressure, the reference diastolic blood pressure, and the standard pulse curve, in the standardization mode; 
 blood flow sound power measuring means for continuously measuring the blood flow sound of the subject and the blood flow sound power in the estimation mode; and 
 second computing means for continuously estimating blood pressure from the blood flow sound power measured continuously, using the correspondence curve in the estimation mode. 
 
     
     
       2. The blood pressure estimation apparatus according to  claim 1 , wherein the first computing means comprises:
 third computing means for creating a reference blood pressure curve which represents a relationship of blood pressure values to standardized time based on the standard pulse curve by bringing the reference diastolic blood pressure value and the reference systolic blood pressure value measured by the blood pressure measuring means into correspondence with a minimum value and a maximum value of standardized pulse pressure values of the standard pulse curve, respectively; 
 fourth computing means for standardizing a time axis in a rising phase of the blood flow sound power waveform at the time corresponding to the time of measurement of the reference diastolic blood pressure value and the reference systolic blood pressure value and creating a reference blood flow sound power curve which represents a relationship of the blood flow sound power to the standardized time; and 
 fifth computing means for creating the correspondence curve which represents a relationship between the blood flow sound power and the estimated blood pressure in a period of common standardized time based on a relationship between the reference blood pressure curve and the reference blood flow sound power curve in the period of common standardized time. 
 
     
     
       3. The blood pressure estimation apparatus according to  claim 2 , wherein
 the first computing means comprises sixth computing means for calculating a systolic blood pressure estimation linear function which approximates the correspondence curve by passing through a point on the correspondence curve corresponding to an end point of the standardized time; and 
 the second computing means inputs a maximum value of the blood flow sound power measured by the blood flow sound power measuring means into the systolic blood pressure estimation linear function and thereby determines estimated systolic blood pressure. 
 
     
     
       4. The blood pressure estimation apparatus according to  claim 3 , wherein
 the sixth computing means calculates a first systolic blood pressure estimation linear function for the subject and a second systolic blood pressure estimation linear function for the subject, where
 according to the first systolic blood pressure estimation linear function for the subject, the maximum value of the blood flow sound power during blood pressure estimation falls below the maximum value of the blood flow sound power at the time of measurement of the reference diastolic blood pressure value and the reference systolic blood pressure value and 
 according to the second systolic blood pressure estimation linear function for the subject, the maximum value of the blood flow sound power during blood pressure estimation rises above the maximum value of the blood flow sound power at the time of measurement of the reference diastolic blood pressure value and the reference systolic blood pressure value; and 
 
 the second computing means determines the estimated systolic blood pressure using the first systolic blood pressure estimation linear function or the second systolic blood pressure estimation linear function depending on variations in the maximum value of the blood flow sound power of the subject during blood pressure estimation. 
 
     
     
       5. The blood pressure estimation apparatus according to  claim 4 , wherein
 the sixth computing means calculates another systolic blood pressure estimation linear function for the subject according to which the maximum value of the blood flow sound power falls to or below a predetermined threshold during the blood pressure estimation; and 
 the second computing means determines the estimated systolic blood pressure using the another systolic blood pressure estimation linear function instead of the first systolic blood pressure estimation linear function if the maximum value of the blood flow sound power of the subject during the blood pressure estimation falls to or below the threshold. 
 
     
     
       6. The blood pressure estimation apparatus according to  claim 5 , wherein
 When the maximum value of the blood flow sound power is equal to the threshold, the another systolic blood pressure estimation linear function has a point of intersection with the first systolic blood pressure estimation linear function and approximates the correspondence curve in a range in which the maximum value of the blood flow sound power is not above the threshold. 
 
     
     
       7. The blood pressure estimation apparatus according to any one of  claims 4  to  6 , wherein
 the first systolic blood pressure estimation linear function is defined by a line which has a slope joining a point existing between 5/12 and 7/12 (both inclusive) of the standardized time and a point existing between 9/12 and 11/12 (both inclusive) of the standardized time when a start point and the end point of the standardized time on the correspondence curve obtained by the fifth computing means are taken as 0 and 1, respectively; and 
 the second systolic blood pressure estimation linear function is defined by a line which has a slope joining that point on the correspondence curve which corresponds to the end point of the standardized time and a point existing between 9/12 and 11/12 (both inclusive) of the standardized time when the start point and the end point of the standardized time on the correspondence curve obtained by the fifth computing means are taken as 0 and 1, respectively. 
 
     
     
       8. The blood pressure estimation apparatus according to any one of  claims 3  to  6 , wherein
 during blood pressure estimation performed by the first computing means, a computational operation for finding the systolic blood pressure estimation linear function is performed a plurality of times to correct the systolic blood pressure estimation linear function. 
 
     
     
       9. The blood pressure estimation apparatus according to any one of  claims 1  to  6 , further comprising:
 mode setting means for setting one of the standardization mode and the estimation mode; 
 information input means for inputting data of at least the reference diastolic blood pressure value, the reference systolic blood pressure value, and the standard pulse curve; 
 information output means for outputting at least estimated blood pressure values; and 
 blood pressure value storing means for storing the reference diastolic blood pressure value and the reference systolic blood pressure value. 
 
     
     
       10. The blood pressure estimation apparatus according to any one of  claims 1  to  6 , wherein
 the pulse wave used to create the standard pulse curve is an average of a plurality of pulse waves obtained from a plurality of unspecified subjects. 
 
     
     
       11. The blood pressure estimation apparatus according to any one of  claims 1  to  6 , wherein
 the pulse wave used to create the standard pulse curve is a pulse wave of the subject whose blood pressure is estimated. 
 
     
     
       12. A blood pressure estimation method for estimating blood pressure of a subject, comprising a standardization mode and an estimation mode, wherein
 the standardization mode comprises: 
 a first step of measuring blood flow sound of the subject and determining a blood flow sound power waveform from the measured blood flow sound, 
 a second step of measuring reference systolic blood pressure and reference diastolic blood pressure of the subject, 
 a third step of preparing, using a processor, a standard pulse curve whose time axis and pulse pressure axis have been standardized such that times and pulse pressures at a start point and an end point of a rising phase of a pulse wave measured in advance will be constant, and 
 a fourth step of determining, using the processor, a correspondence curve which represents a relationship between blood flow sound power and estimated blood pressure from relationships among the blood flow sound power waveform, the reference systolic blood pressure, the reference diastolic blood pressure, and the standard pulse curve in the standardization mode; and 
 the estimation mode comprises: 
 a fifth step of continuously measuring the blood flow sound of the subject and the blood flow sound power, and 
 a sixth step of continuously estimating, using the processor, blood pressure from the blood flow sound power measured continuously, using the correspondence curve in the estimation mode. 
 
     
     
       13. The blood pressure estimation method according to  claim 12 , wherein the fourth step comprises:
 a step of creating a reference blood pressure curve which represents a relationship of blood pressure values to standardized time based on the standard pulse curve by bringing the reference diastolic blood pressure value and the reference systolic blood pressure value into correspondence with a minimum value and a maximum value of standardized pulse pressure values of the standard pulse curve, respectively; 
 a step of standardizing a time axis in a rising phase of the blood flow sound power waveform at the time corresponding to the time of measurement of the reference diastolic blood pressure value and the reference systolic blood pressure value and creating a reference blood flow sound power curve which represents a relationship of the blood flow sound power to the standardized time; and 
 a step of creating the correspondence curve which represents a relationship between the blood flow sound power and the estimated blood pressure in a period of common standardized time based on a relationship between the reference blood pressure curve and the reference blood flow sound power curve in the period of common standardized time. 
 
     
     
       14. The blood pressure estimation method according to  claim 13 , wherein
 the fourth step calculates a systolic blood pressure estimation linear function which approximates the correspondence curve by passing through a point on the correspondence curve corresponding to an end point of the standardized time; and 
 the sixth step inputs a maximum value of the blood flow sound power measured by the fifth step into the systolic blood pressure estimation linear function and thereby determines estimated systolic blood pressure. 
 
     
     
       15. The blood pressure estimation method according to  claim 14 , wherein
 the fourth step calculates a first systolic blood pressure estimation linear function for the subject and a second systolic blood pressure estimation linear function for the subject, where according to the first systolic blood pressure estimation linear function for the subject, the maximum value of the blood flow sound power during blood pressure estimation falls below the maximum value of the blood flow sound power at the time of measurement of the reference diastolic blood pressure value and the reference systolic blood pressure value and according to the second systolic blood pressure estimation linear function for the subject, the maximum value of the blood flow sound power during blood pressure estimation rises above the maximum value of the blood flow sound power at the time of measurement of the reference diastolic blood pressure value and the reference systolic blood pressure value; and 
 the sixth step determines the estimated systolic blood pressure using the first systolic blood pressure estimation linear function or the second systolic blood pressure estimation linear function depending on variations in the maximum value of the blood flow sound power of the subject during blood pressure estimation. 
 
     
     
       16. The blood pressure estimation method according to  claim 15 , wherein
 the fourth step calculates another systolic blood pressure estimation linear function for the subject according to which the maximum value of the blood flow sound power falls to or below a predetermined threshold during the blood pressure estimation; and 
 the sixth step determines the estimated systolic blood pressure using the another systolic blood pressure estimation linear function instead of the first systolic blood pressure estimation linear function if the maximum value of the blood flow sound power of the subject during the blood pressure estimation falls to or below the threshold.